People cheat because they are insecure. The attention and affection of another makes them feel good. It helps them believe that they are a worth while person, these people have poor self esteem. They seek attention from someone in addition to the attention of their partner because they are not convinced that they deserve love from anyone, they hope that by finding more and more partners, they will some day be convinced of their own self worth.
The important thing to remember is people do not cheat because their partner does anything wrong, it is a problem with the cheater.